The pump of this invention is in the same field as the pumps shown in the following U.S. Pat. Nos. 2,187,684; 2,636,441; 2,787,225; 3,469,532; 3,502,029; 3,945,772; 4,487,340; 4,762,474; and 6,102,676. Of particular interest is U.S. Pat. No. 2,787,225, which is directed to a lance pump sold by Lincoln Industrial Corporation of St. Louis, Mo., under the designation Series 20. Although lance pumps such as those identified above have been commercially successful, there is a need for a pump that provides a selectively variable output pressure and reduces disassembly and assembly complexity.